Lead-lag filter arrangement for electro-pneumatic control loops

ABSTRACT

A lead-lag input filter is connected ahead of a positioner feedback loop having one or more valve accessories, such as a volume booster or a QEV, to overcome slow dynamics experienced by the accessories when receiving low amplitude change control or set point signals. A user interface is connected to the lead-lag input filter and enables an operator or other control personnel to view and change the operating characteristics of the lead-lag input filter to thereby provide the control loop with any of a number of desired response characteristics.

REFERENCE TO RELATED APPLICATIONS

This disclosure is entitled to the benefit of the filing date of U.S.Provisional Application Nos. 60/620,537, filed Oct. 20, 2004, and60/652,546, filed Feb. 14, 2005, as to all subject matter commonlydisclosed therein.

FIELD OF THE DISCLOSURE

This disclosure relates generally to the field of servo controllers foruse in logical processes or control loops and, more particularly, to theaugmentation of electro-pneumatic control loops and other logicalprocesses for improvement of performance of control valves and pneumaticactuator accessories.

BACKGROUND

Electro-pneumatic control systems are increasingly being employed withprocess control devices, such as valve actuators and piston actuators,in order to provide better or more optimal control of fluid within aprocess plant. Some such electro-pneumatic control systems include oneor more accessories for controlling valve and piston actuators such asvolume boosters and quick exhaust valves (QEVs). A volume booster, whichis typically coupled to a pneumatic actuator for a valve, increases therate of air supplied to the pneumatic actuator, or increases the rate ofair exhausted from the pneumatic actuator. This increased air movementamplifies the actuator stroke speed, thereby increasing the speed atwhich the actuator is able to stroke the valve plug toward its open orclosed position, and thus enables the valve to respond more quickly toprocess fluctuations. Similar to volume boosters, QEVs increase thespeed at which an actuator is able to stroke a valve toward an open orclosed position.

Currently, volume boosters are utilized with pneumatic actuators in amanner that makes the actuators move very slowly in response to verysmall set point or control signal changes. In particular, some volumeboosters are designed with a built-in dead band to actually prevent thevolume booster from becoming active in response to small amplitudechange control signals. While some volume boosters have small dead bandsat the lower amplitude signal range, these volume boosters still movevery slowly in response to small amplitude signal changes, becoming fastonly in response to larger amplitude input signals.

DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 is a block diagram of an electro-pneumatic control systemaugmented with a lead-lag input filter;

FIG. 2 is an example screen display generated by a user interfaceroutine of an electro-pneumatic control system, such as that shownschematically in FIG. 1, illustrating travel set point plotted againsttime, and lead-lag filter response plotted against time, when thelead-lag input filter is engaged;

FIG. 3 is an example screen display generated by a user interfaceroutine of an electro-pneumatic control system, such as that shownschematically in FIG. 1, illustrating travel set point plotted againsttime, and lead-lag filter response plotted against time, when thelead-lag input filter is disengaged;

FIG. 4 is an example screen display of a menu enabling a user to selecta stimulus source for the lead/lag filter of the control loop and toinput values in data entry fields when such fields are enabled;

FIG. 5 is a flow chart diagramming actions performed and informationdisplayed as a result of various inputs in a user interface of anelectro-pneumatic control system;

FIG. 6 is a flow chart diagramming the status of various input controlsof a user interface in response to particular filter type selections;and

FIG. 7 is an example screen display of a menu enabling a user to selectamong various instrument control settings, including a setting “RemoteTuning”.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Generally speaking, a lead-lag input filter is provided ahead of apositioner feedback loop in conjunction with one or more valveaccessories, such as a volume booster or a QEV, to overcome slowdynamics experienced by the accessories when receiving low amplitudechange control or set point signals. Additionally, a user interfaceenables an operator or other control personnel to view and change theoperating characteristics of the lead-lag input filter to therebyprovide the control loop with any of a number of desired responsecharacteristics. Through manipulation of the ratio of lead-to-lag of thelead-lag input filter, a process parameter, such as displacement ortravel of a valve stem, may be controlled, and in particular, finetuned.

FIG. 1 illustrates a control loop 40, such as an electro-pneumaticcontrol loop or other logical process, having a lead-lag filter 20connected to the input thereof. In particular, a reference controlsignal 10, such as a 4-20 mA set point signal or control signalgenerated by a process controller or user interface, is applied to theinput of the lead-lag input filter 20 which operates on the referencesignal (which can be a set point or other control signal) to provide afiltered output 50 (also called a travel set point signal) to a summer30 associated with the electro-pneumatic control loop 40. As illustratedin FIG. 1, the summer 30 compares the valve travel with the travel setpoint signal 50 to generate an error signal, which is provided to anamplifier or gain unit 90 (called a forward path gain unit) whichapplies a gain K. The output of the forward path gain unit 90 isprovided to a further summer 94 which sums (in this case, subtracts) avelocity feedback gain developed by a gain unit 95 and a minor loopfeedback gain developed by a gain unit 105 from the output of theforward path gain unit 90. The output 110 of the summer 94 is providedto a current-to-pressure (I/P) transducer 80 which develops and providesa pneumatic or pressure signal to a pneumatic relay 85. As illustratedin FIG. 1, a measurement of the relay position 100 is provided to thegain unit 105 and is used to develop the minor loop feedback gain.

The pneumatic output of the relay 85 is provided to the volume boosteror QEV 65. This pneumatic signal is used to control the valve actuatorof an actuator 55 associated with a valve 60. As illustrated in FIG. 1,the measured valve travel of the valve plug, or the position of thevalve stem with which the valve plug is associated, is provided to thesummer 30 for comparison to the travel set point signal, as well as tothe velocity feedback gain unit 95 to develop the velocity feedbackgain. At least one sensor (not shown) is employed to detect the measuredvalve travel of the valve plug or the position of the valve stem.

Generally speaking, the transfer function and operation of lead-laginput filter 20 is configurable via a user interface 107. In particular,a technician can remotely adjust the travel set point signal 50 fordriving the pneumatic actuator 55 and the control valve 60, or otherdevice controlled by the electro-pneumatic control loop 40, by adjustingparameters of the lead-lag filter 20. The user interface 107 may beprovided to enable remote monitoring of, control of, or communicationwith the electro-pneumatic control loop 40 from a remote location orfrom a location in the immediate vicinity of the control loop 40.

During operation, the lead-lag filter 20 will generally provide a largeamplitude, but short duration, spike at the beginning of any step changein the received reference signal 10, which allows the valve 60 to movein smaller steps. Additionally, a fast decay rate (which translates to asmall lag time) is provided in the filter response to mitigate overshootfor larger steps.

While a distributed control system (DCS) typically updates at afrequency on the order of 1 Hz or slower, a positioner (within thecontrol loop 40) can update at a frequency of 100 Hz or more. As aresult, the response time provided by the lead-lag filter 20 in serieswith the positioner can be on the order of 100 ms, which is much fasterthan can be provided by the control dynamics of the DCS alone.

Additionally, the lead-lag filter 20 can provide inherent protectionagainst over driving the valve plug of the valve 60 into the valve seator into the upper travel stop. In particular, algorithms or controlroutines can be implemented within or as part of the filter 20 to clipthe valve's response near a valve seat or a travel stop, and therebyprevent the lead-lag filter 20 from bouncing the valve plug of the valve60 off of the valve seat or an upper travel stop.

Still further, as will be understood with respect to FIGS. 2 and 3, theoperating characteristics of the lead-lag filter 20 can be easilyadjusted using the user interface 107, which may be stored in a computerand operably coupled to the control loop 40 and one or more displayscreens. Because many processes that use large actuators with complexaccessory configurations generally require complicated and highlycustomized control algorithms to control the process loop, operators aretypically reluctant to modify the process controller by adding dynamicswithin the control routine. Instead, operators generally prefer toeffect or change dynamics at the valve level. The lead-lag filter 20,which can be modified to vary the process dynamics at the valve or looplevel, provides the operator with just such control.

As illustrated in FIG. 1, the lead-lag input filter 20 is preferablyimplemented in combination with a user interface 107, such as a computerprogram with user-friendly, real-time graphics. One or more routines andone or more processors in operable communication with the user interface107, the lead-lag input filter 20, and one or more devices or componentswithin the control loop 40 may be employed to implement thefunctionality and features disclosed herein.

The user interface 107 is preferably implemented in communication with agraphical user interface (GUI) to facilitate a user's interaction withthe various capabilities provided by the user interface 107 and lead-laginput filter 20. The GUI may include one or more software routines thatare implemented using any suitable programming languages and techniques.Further, the software routines making up the GUI may be stored andprocessed within a single processing station or unit, such as, forexample, a workstation, a controller, etc., such as in a control roomwithin a process control plant or a central control room facility forone or a number of geographically remote process control plants, or,alternatively, the software routines of the GUI may be stored andexecuted in a distributed manner using a plurality of processing unitsthat are communicatively coupled to each other.

Preferably, but not necessarily, the GUI may be implemented using afamiliar graphical windows-based structure and appearance, in which aplurality of interlinked graphical views or pages include one or morepull-down menus that enable a user to navigate through the pages in adesired manner to view and/or retrieve a particular type of information.The features and/or capabilities of the user interface 107 describedherein may be represented, accessed, invoked, etc. through one or morecorresponding pages, views or displays of the GUI. Furthermore, thevarious displays making up the GUI may be interlinked in a logicalmanner to facilitate a user's quick and intuitive navigation through thedisplays to retrieve a particular type of information or to accessand/or invoke a particular capability of the user interface 107 andlead-lag input filter 20.

An example of such a GUI is generally depicted in a display 120illustrated in FIG. 2. As depicted in FIG. 2, the display 120graphically depicts the filter output or travel set point signal 50 andthe position feedback, utilizing, for example, data collected from theactuator feedback signal 70 or the relay position feedback signal 100.The feedback signals 70, 100 vary proportionally in response to changesin a process parameter with which they are associated, in this case theposition of the actuator 55 or the relay 85, so graphically depictingchanges in the feedback signals 70, 100 provides an accurate indicationof actual variation in valve stem position. Such real-time graphicsallows the control valve 60 to be tuned remotely and providesquantifiable results. Additionally, remote tuning of the control valveloop via the user interface 107 significantly reduces maintenance costsby avoiding physical maintenance visits to individual control valves.

A control room with one or more computer terminals for accessing theuser interface 107 may be provided in the geographic vicinity of thevalves or loops to be controlled. Alternatively, satellitecommunication, telephone lines, coaxial cable, Ethernet, fiber opticcable connections, an intranet, the Internet, or other long distancecommunication technology may be employed to provide remote access to theuser interface 107 at geographically distant locations. A centralcontrol facility may be provided in which one or more computer terminalsfor accessing the user interfaces 107 associated with valves or loopsprovided with lead-lag filters 70 in a plurality of locations separatedby long distances from the central control facility. As explained ingreater detail below, the user interface 107 is provided with a plotallowing the operator or technician to predict or view the filterresponse when particular settings are selected for varioususer-adjustable parameters of the lead-lag input filter 20.

While there is inherent delay when signals or data are transmitted viaone or a combination of the various communication technologiesespecially over long distances, the user interface 107 can be employedin a manner to adjust for such delays, provided the extent of the delaysare known or can be calculated or determined. For example, the userinterface 107 may provide the user or operator with the option ofimplementing a particular set of adjustments to the user-adjustableparameters of the lead-lag input filter 20 which the user or operatorhas first plotted using the predicted response capabilities of the userinterface 107, discussed in more detail below. If the new set ofadjustments is to be implemented for a valve or loop in a distantlocation at a time selected by the user or operator, the user interface107 may factor the delay into a calculation of the timing for sendingactual signals to the lead-lag input filter 20 of a particular valve orloop. For instance, if the user or operator wants the new set ofadjustments to be implemented in 10 seconds, and there is a known orcalculated delay of 0.5 second, the actual signal to the lead-lag inputfilter 20 may be sent in 9.5 seconds. This assumes the user or operatoris receiving and displaying in real time the filter output and travelfeedback data concerning the actual control valve or control loop towhich the lead-lag input filter 20 has been added.

Using a computer software program for the control of parametersassociated with a control valve, such as the AMS ValveLink® Softwareprogram, available from the Fisher Controls division of Emerson ProcessManagement, the user interface 107 may be configured to displayreal-time filter output and travel feedback data from the control valveor other device with which the lead-lag input filter 20 is employed.Additional data may also be displayed, such as reference signal to thedevice. For example, as illustrated in FIG. 2 by the graph 130, the userinterface 107 may plot on the GUI the real-time travel set point (“TvlSet Pt”) and travel feedback data (“Tvl”), displayed as percentages (%),against time to enable an operator to easily view the response of thecontrol valve to changes in the reference signal.

The improved control achieved by using the lead-lag filter 20 at lowamplitudes can be appreciated by comparing the plot 130, shown in thegraphics display 120 shown in FIG. 2, reflecting real-time data for thetravel set point 50 and the travel feedback 79 collected while thelead-lag filter 20 is engaged, to the plot 135 shown in the graphics 140displayed in FIG. 3, reflecting data collected while the lead-lag filteris turned off or disengaged after the 0:02:12 time mark, where the timesdisplayed on the horizontal axis of the plot are in hours, minutes, andseconds. Here, it can be seen that, without the lead-lag filter 20, theresponse of the valve 60 deteriorates in and slows as a result of asimple step change in the travel set point (reference) signal. Real-timegraphics, such as those illustrated in FIGS. 2 and 3, are particularlyadvantageous for tuning the lead-lag input filter 20, given thesensitivity and complexity associated with the valve dynamics, even atlow amplitudes.

Referring again to FIG. 2, for ease of operation, tuning coefficientsassociated with the lead-lag input filter 20 may be represented in thedisplay 120 of the user interface routine using a filter response plot150. Additionally, the tuning coefficients (and thereby the transferfunction) associated with the lead-lag filter 20 may be changed usingone or more virtual interface controls 200, depicted in FIG. 2 asgraphical representations of slider bars 210, 220, and 230. A controloperator or technician may manipulate the slider bars 210, 220, and 230using, for example, a computer input device (not shown) such as a mouse,knob, trackball, keyboard, touch-screen monitor, voice-activation, orstylus pad to thereby change the transfer function or dynamics of thelead-lag input filter 20. Of course, this list of computer input devicesis intended to be exemplary only, and other input devices may likewisebe used to manipulate the sliders 210, 220, and 230. Also, the virtualinterface controls 200 may alternatively be graphically represented by,for example, dials (not shown) or other graphics. Additionally, asillustrated in FIG. 2 at the areas 205, 207, 209 to the left of thesliders 210, 220, 230, the filter coefficients or ratios selected by thesliders 210, 220 and 230 may be displayed in numerical form, and buttons214 and 216, shown in the area designated 212 of the display 120, may beused to apply the current settings or to reset the current setting ofthe lead-lag filter 20.

Valid values for the lag time filter coefficient 205 include 0.00 (whichresults in bypassing the filter), and values in a range from 0.10 to10.00 seconds. Preferably, the range of lag time filter coefficients 205is shown in a logarithmic scale on the plot 130 of the display 120,inasmuch as most lag time filter coefficients are selected in a rangefrom 0.10 to 2.00 seconds.

Valid values for the lead time to lag time ratio in the openingdirection 207, and lead time to lag time ratio in the closing direction209, range from 0.0 to 2.0, and are shown in a linear scale on thedisplay 120.

As illustrated in FIG. 2, the slider 210 adjusts the lag time, whichdetermines the decay rate of the filter response. The larger the lagtime, the slower the lead-lag input filter 20 returns its output to thereference signal 10. The slider 220 of FIG. 2 adjusts the ratio of thelead time to the lag time in the opening direction. The slider 230 ofFIG. 2 adjusts the ratio of the lead time to the lag time in the closingdirection. This ratio determines the initial response of the lead-laginput filter 20. As indicated above, the lead-lag filter 20 is generallyconfigured to provide a large amplitude, but short duration, spike inthe travel set point 50, which allows the valve 60 to move in smallersteps. A fast decay rate (which translates to a small lag time) alsomitigates overshoot for larger steps because the valve 60 tends to slewallowing the filter response to decay away completely before the valve60 gets close to the set point.

Additionally, the filter response graph 150 (FIG. 2) provides theoperator or technician with the ability to predict or view the filterresponse when particular settings are selected for the varioususer-adjustable parameters, such as lag time and ratio of lag time tolead time. The filter response graph 150 of FIG. 2 illustrates thepredicted response of the iead-lag filter 20 to a unit step changebefore the parameters changes are applied to the lead-lag filter 20 tothereby enable the operator or technician to view a graphicalrepresentation of the predicted filter response before the dynamics ofthe control system are actually adjusted. Thus, there is a virtual ratioof lead-to-lag that an operator may manipulate in order to generate apredicted response of a process parameter to be controlled or tuned, andthat predicted response is displayed on a display associated with theuser interface 107. A similar filter response graph 155 in FIG. 3displays the response when the lead-lag input filter 20 is turned off ordisengaged.

Additionally, an operator may use the selection buttons in the area 228of the user interface display 120 of FIG. 2 to configure the lead-lagfilter 20 to be turned off or disengaged, to adjust just the lag elementof the response, to adjust or select both the lag and the lead/lag ratioof the filter response, or to enable asymmetric lead/lag ratios, i.e.where there is a non-zero lag time coefficient, and the coefficients forthe lead time to lag time ratio in the opening direction differs fromthe lead time to lag time ratio in the closing direction. When the lagtime coefficient is zero, and there are non-zero, but identical leadtime to lag time ratio coefficients, the lead-lag dynamics aresymmetrical.

By storing collected and predicted data displayed in the plots 130, 150in a buffer or readable memory of or operatively coupled to a computer,the plots 130, 150 may be paused, rewound, and replayed at theoperator's or technician's convenience, or for future quality control,efficiency, and optimization purposes, educational purposes, regulatorycompliance purposes, or other purposes.

Control mechanisms, such as the graphically depicted buttons 310, 315,320 and slider 330 shown at the top of the display 120 of the userinterface 107, may be manipulated with an appropriate computer inputdevice, such as those listed above, to control a latency period, ordelay, between the predicted response depicted in the filter responsegraph 150 and real world application of the settings to effect actualadjustment of the control system dynamics. In the event an operatordetermined that the predicted response to a particular adjustment or setof adjustments to the tuning coefficients by manipulation of one or moreof the virtual interface controls 200 was an undesired result, theoperator can manipulate the graphically depicted buttons 310, 315, 320or the slider 330 to increase the latency period, and readjust thetuning coefficients until a desired result is depicted in the filterresponse graph 150, preventing the undesired result from ever occurringin the actual, real world control system.

Other operations, such as printing, may be performed by a technician'sor operator's selection of other graphically depicted buttons 335, 340,345, 350, 355, 360 on the display 120.

The user interface allows the stimulus for tuning the valve 60 to beapplied externally (e.g., through a DCS) or “internally” with a computersoftware program such as ValveLink® configured to send a digital stepcommand to the positioner. Using an external stimulus, the usermanipulates the 4 mA-20 mA input signal and the valve respondsaccordingly. In addition, the lead-lag filter 20 may be implementedeither directly in a device, such as in a valve positioner, or in adistributed control system connected to the device, e.g., in acontroller. Generally speaking, the lead-lag filter 20 may beimplemented as a digital control program or routine stored in a computerreadable memory and executed on a processor, but may be implemented asan analog filter as well.

The user interface 107 may be provided with an option screen allowingthe user to readily select an external stimulus or an internal stimulus.When the external stimulus is selected, operator adjustment of theadjustable interface controls alters at least one tuning coefficientassociated with the lead-lag filter to cause modifications to thereference control signal. When the internal stimulus is selected, theadjustable interface controls are at least partially disabled, such thatthe disabled interface controls no longer alter tuning coefficientsassociated with the lead-lag filter. Instead, the tuning coefficients ofthe lead-lag filter are modified in response to a controller includingprogramming adapted to cause predetermined modifications to thereference control signal.

For instance, as shown in FIG. 4, a menu is provided from which a usermay select either “External Stimulus” or “ValveLink Stimulus (SquareWave)”, which will be understood to be an internal stimulus. Selectingthe internal stimulus option enables the user to enter values for thedata entry fields “Nominal Set Point (%)”, “Step Size (%)”, and “StepHold Time (sec)”. When “External Stimulus” is selected, these data entryfields become disabled. When the internal stimulus option is selected,the program may be configured to automatically populate the data entryfields with initial default values, such as the following:

DATA ENTRY FIELD DEFAULT VALUES Nominal Set Point 50% Step Size 15% StepHold Time 8 seconds

FIG. 5 is a flow chart diagramming the results performed and displayedon the user interface, depending on whether an external stimulus or aninternal stimulus is selected. Warning messages or other alerts arepreferably displayed before initiating control valve operation to remindthe user that, in the case of selection of an external stimulus, thevalve will track the set point, and in the case of selection of aninternal stimulus, the internal stimulus will cause the valve to move.If the internal stimulus option is selected, the set point valuepreferably ramps to the value entered for the nominal set point at 10%per second before the step sequence is initiated.

FIG. 6 is a flow chart diagramming the status of various input controlsof the user interface in response to particular filter type selections.For instance, when an asymmetric lead-lag filter type is selected, theuser interface is configured to enable the user interface control formanipulating lag time. The user interface also is configured to enablethe user interface control for manipulating the opening lead-lag ratioand the closing lead-lag ratio. Conversely, if a symmetric or simplelead-lag filter is selected, an initial value from a database isprovided in a data entry field of the user interface for the openinglead/lag ratio, the user interface is configured to enable the userinterface controls for manipulating the lag time and opening lead/lagratio, but the user interface control for setting the closing lead/lagratio is disabled.

Still further, as indicated above, the filter 20 may be provided with anautomatic reset of the lead-lag filter dynamics to prevent the filter 20from inadvertently activating above or below a cutoff. In particular,the lead-lag input filter 20 may, in some situations, have theundesirable capability to bounce the valve plug of the valve 60 off theseat or off of a travel stop. This is a particularly difficult problembecause positioners typically have built-in travel cutoffs that fullysaturate the I/P transducer 80 when set point approaches 0% or 100%. Fora Fisher DVC6000 digital valve controller, the problem associated withthe use of lead-lag filters at the high or low range of the valve isavoided by establishing travel cutoffs using default values of 0.5% and99.5%, meaning that if the reference signal or set point falls below0.5% or exceeds 99.5%, the servo controller is bypassed and the I/Ptransducer 80 is either saturated at full supply or vented to theatmosphere, depending on the required saturation state. As a result,during normal throttling operation the lead-lag input filter 20 shouldnot trip a cutoff.

A pseudo computer programming code provided below demonstrates anexample computer program code implementation that may be used to assurethat a controller associated with or that implements the lead-lag inputfilter 20 prevents cutoffs from being tripped. In this case, thelead-lag input filter 20 is bypassed and the dynamics are reset if theoutput of the filter 20 exceeds a predefined limit near the cutoffvalue, such as at 0.5% or 99.5%, although other values can be used aswell.

//------------------------------------------------------------------------// Begin lead-lag filter//------------------------------------------------------------------------//--- Prefilter stage --- if((r >= filter_limit_high) ||(r <=filter_limit_low) || (lag_time == 0.0)) {   x = r; // bypass filter whenin or near cutoffs } else // --- Filter stage --- {   x = a * (r_old −x_old) + x_old + b * (r − r_old);   // check filter output to make surewe do not bump into cutoffs   if(x >= filter_limit_high)    x =filter_limit_high;   else if (x<= filter_limit_low)    x =filter_limit_low; } // --- Post filter stage --- x_old = x; // updateold values r_old = r;//-----------------------------------------------------------------------// End lead-lag filter//-----------------------------------------------------------------------

In one embodiment, the lead-lag input filter 20 may be implemented withfour states, or stages, of execution including a prefilter stage, afilter stage, a post-filter stage, and an initial condition stage. Inthe prefilter stage, the filter 20 checks to determine if the referencesignal 10 has exceeded a predefined upper limit, has dropped below apredefined lower limit, or if the filter 20 has been turned offaltogether. When the reference signal 10 exceeds the predefined upperlimit or drops below the predefined lower limit (or the filter 20 isturned off or disengaged via the user interface 107), the lead-lag inputfilter 20 bypasses processing of the reference signal and, instead,provides the reference signal 10 directly to the input 30 of theservo-loop. As indicated above, the predefined upper and lower limitsare preferably set so that output of the lead-lag input filter 20 willnot trip a cutoff or hit a hard stop in the actuator.

The following pseudo computer programming code demonstrates one mannerin which a controller associated with the lead-lag input filter 20 maybe programmed so as to set the upper and lower filter limits todesirable threshold levels:

Filter_limit_high=min((ivp_cutoff high−high_cutoff_deadband),(100%−high_cutoff_deadband))

Filter_limit_low=max((ivp_cutoff_low+low_cutoff deadband),(0%+low_cutoff_deadband))

These limits may be calculated in firmware and are calculated every timethe input characteristic, lower travel cutoff, or upper travel cutoffvalues are changed. Moreover, because the cutoff processing algorithm isdownstream of the characterizer, these limits are passed through aninverse characteristic (with x- and y-data vectors reversed) so that thecharacterized limits are below the cutoff thresholds.

In the filter stage, the lead-lag input filter 20 operates as a standarddiscrete time filter. Generally speaking, the lead-lag input filter 20may be represented as having two coefficients, “a” and “b.” Coefficient“a” is the coefficient for the lag contribution and coefficient “b” isthe coefficient for the ratio of the lead time to lag time, which may beexpressed formulaically as: τ_(lead)/τ_(lag). To prevent the lead-laginput filter 20 from activating a cutoff or hitting a hard travel stop,the output of the filter 20 is preferably reset to the same upper andlower values used in the prefilter stage. During the filter stage orstate, the filter 20 applies the filter coefficients (ratio) to thereference signal in any known or desired manner to create the filteredinput signal for the servo-loop.

During the post-filter stage, the previous values used in the filtercalculations are updated based on new inputs from the user interface orfrom the servo-loop. Finally, during the initial conditions stage, whichoccurs for example when an instrument is started up, the initialconditions of the lead-lag input filter 20 are set to the present inputreference value. Of course, in order to provide inverse dynamics tononlinearities in the pneumatics, filter coefficients may be separatelyadjusted for the opening direction and the closing direction of acontrol valve 60.

In a preferred embodiment, the lead-lag input filter's result, i.e. theeffect of the lead-lag input filter 20 on the set point or the valveinput signal, is given by the formula:

(τ₁ s+1)/(τ₂2s+1)

By adjusting the values of τ₁ and τ₂, the ratio is changed, effectingpure lag, pure lead, or some combination of lead and lag. When appliedto a control valve, the resulting ratio correlates to the amount ofovershoot that the lead-lag filter will provide. Thus, in differentvalve performance scenarios, the operator may use the user interface 107to adjust the ratio to achieve desired alterations. For instance, if itis desired for the lead-lag input filter 20 to produce pure lag, thenτ₁s is set to zero, producing a result of 1/(τ₂s+1). In a control loop,when pure lag is generated by the lead-lag input filter 20, error isdriven towards zero. As a result, the position of the control valve stemwith which the lead-lag filter 20 is employed, or other process variablebeing controlled, will creep to the travel set point 50.

If it is desired for the lead-lag input filter 20 to produce pure lead,then τ₂s is set to zero, producing a result of (τ₁s+1)/1. In a controlloop, this provides anticipatory control, by correcting for error priorto occurrence of such error. When plotted, the operator of the userinterface 107 would see positive phase with respect to the controlledelement.

So long as the lead-lag ratio is greater than 1.0, the initial leadresponse will dominate. If the lead-lag ratio is 2, there is an initiallead response of 2.0, as a result of which any correction in theposition of the control valve stem or shaft is substantially reduced,prior to error propagating through the control circuit, and will thengradually move the control valve stem position, or other processvariable being controlled, to the travel set point 50. If the lead-lagratio is less than 1.0, then the lag correction will dominate.

By recognizing the change in performance in various valve performancescenarios resulting from various possible lead-lag ratios, operators maybecome easily adept at fine tuning process parameters and correcting forerrors, and may easily optimize control valve performance.

It will be recognized that additional components may advantageously beprovided that benefit from the use of a lead-lag filter 20. For example,feedforward components may be provided which are adapted to respond todata including the reference signal 10, velocity of the reference signal10, and acceleration of the reference signal 10.

The display 120 of the user interface 107 is preferably accessed throughone or more menu screens, such as a pull-down menu screen captioned“Instrument Setup” as shown in FIG. 7. The menu screen(s) preferablyprovide adequate indicia to inform the user that the control valve loopmay be remotely tuned. For instance, a menu option in FIG. 5 reads“Remote Tuning”. When selected, the user may select “EnhancedStabilize/Optimize Lead-Lag Input Filter”.

The lead-lag input filter 20 may be implemented in any number ofdifferent types of servo-loops. Thus, while the lead-lag input filter 20is illustrated in FIG. 1 as being used in one type of electro-pneumaticcontrol system comprising a high-gain, closed-loop servo controller usedto set stem or shaft position on control valves, it could be used inother control systems or control loops as well. For example, anotherapplication in which a lead-lag filter associated with a set point iseffective is in combination with ball valves where shaft windup betweenthe actuator and the plug introduces dead band in flow control. Shaftwindup may be overcome by briefly over driving the actuator and allowingthe ball to move to the desired location. Because this is an open-looptechnique, the response is not perfect, but a considerably betterresponse is obtained than without a lead-lag filter.

Still further, there are various techniques available to improveperformance by driving the servo to set point faster than what wouldnormally be achieved by closed loop compensation alone, without changingclosed loop dynamics. Augmenting the feedback controller with a lead-lagfilter on the set point is one such technique, while other techniquesinvolve augmenting the controller with set point velocity feedforwardelements. The lead-lag filter could be used in these situations as well.

The technique in which a feedback controller is augmented with alead-lag input filter is particularly useful in applications in whichaccessories for increasing actuator stroke speed, such as volumeboosters and QEVs, are used. In order to compensate for slow dynamics atlow amplitude changes, a lead-lag filter may be used to over drive theset point for a brief amount of time, so as to engage volume boosterseven at lower amplitudes, such as amplitudes at which conventionalvolume booster arrangements would not be effectively activated due tolow dead bands.

While the lead-lag filter 20 may be implemented in a desired manner,including in software and hardware or firmware, when implemented insoftware, the software routines discussed herein may be stored in anycomputer readable memory such as on a magnetic disk, a laser disk, orother storage medium, in a RAM or ROM of a computer or processor, suchas an application specific integrated circuit (ASIC), a standardmulti-purpose CPU or other hard-wired device, etc. Likewise, thesoftware may be delivered to a user or a process control system via anyknown or desired delivery method including, for example, on a computerreadable disk or other transportable computer storage mechanism or overa communication channel such as a telephone line, the Internet, etc.(which are viewed as being the same as or interchangeable with providingsuch software via a transportable storage medium).

While certain embodiments have been described herein, claims to thedisclosed invention are not intended to be limited to these specificembodiments.

1. A method for controlling a process parameter of a control loopcomprising: providing a reference control signal at an input to acontrol loop; providing a lead-lag filter in communication with thereference control signal prior to amplification of the reference controlsignal; providing a user interface in operable communication with thelead-lag filter, said user interface facilitating remote manipulation ofa ratio of lead-to-lag produced by the lead-lag filter; and operatingthe user interface to remotely manipulate the ratio of lead-to-lag ofthe lead-lag filter to produce an alteration in the process parameter tobe controlled.
 2. The method of claim 1, wherein operating the userinterface includes adjusting at least one tuning coefficient associatedwith the lead-lag filter by manipulating at least one virtual interfacecontrol provided on a display associated with the user interface.
 3. Themethod of claim 2, and displaying data associated with the processparameter to be controlled.
 4. The method of claim 3, wherein the datais displayed on the display associated with the user interface.
 5. Themethod of claim 1, and manipulating a virtual ratio of lead-to-lag togenerate a predicted response of the process parameter to be controlled,and displaying the predicted response on a display associated with theuser interface.
 6. The method of claim 1, wherein the reference controlsignal is a 4-20 mA control signal.
 7. A system for tuning a processparameter of a control loop comprising: a lead-lag input filter incommunication with an input to the control loop; a controller applyingan unamplified reference control signal to an input of the lead-laginput filter; a user interface in operable communication with thelead-lag filter, said user interface including at least one adjustableinterface control, wherein adjustment of each of said at least oneadjustable interface controls alters at least one tuning coefficientassociated with the lead-lag filter.
 8. The system of claim 7, whereinthe user interface further includes a display for monitoring a processparameter affected by alteration of the at least one tuning coefficient.9. The system of claim 8, wherein the control loop includes at least onefeedback signal that varies with changes in the process parameter. 10.The system of claim 8, wherein the user interface includes a display onwhich variations in the at least one feedback signal are graphicallydisplayed.
 11. The system of claim 7, wherein the user interface furtherincludes a display for a monitoring a predicted response of the processparameter in response to adjustments of each of the at least oneadjustable interface controls.
 12. The system of claim 11, wherein theuser interface is provided with at least one control mechanism tocontrol a latency period between the predicted response of the processparameter to adjustments of each of the at least one adjustableinterface controls, and application of the adjustments of each of the atleast one adjustable interface controls to the lead-lag filter to effectan actual response of the process parameter.
 13. The system of claim 7,wherein said user interface is provided in a location remote from thelead-lag input filter.
 14. A system for tuning the response of a controlvalve comprising: a control loop including a valve controller, acurrent-to-pressure transducer, a control valve, and a valve actuator inoperable communication with a valve plug of the control valve; alead-lag filter in communication with an input to the control loop; anda process controller supplying an unamplified reference control signalto an input of the lead-lag filter.
 15. The system of claim 14, furthercomprising a user interface in operable communication with the lead-lagfilter, said user interface including at least one adjustable interfacecontrol, wherein adjustment of each of said at least one adjustableinterface controls alters at least one tuning coefficient associatedwith the lead-lag filter.
 16. The system of claim 15, wherein the userinterface is located at a remote location from the lead-lag filter. 17.The system of claim 15, wherein the user interface communicates with thelead-lag filter through at least one of a group of telephone lines,satellite transmission, coaxial cable, Ethernet, fiber optic cable, andthe Internet.
 18. The system of claim 15, wherein the user interfacefurther includes a display for a monitoring a predicted response of aposition of the valve plug of the control valve in response toadjustments of each of the at least one adjustable interface controls.19. The system of claim 18, wherein the user interface is provided withat least one control mechanism to control a latency period between thepredicted response of the position of the valve plug of the controlvalve to adjustments of each of the at least one adjustable interfacecontrols, and application of the adjustments of each of the at least oneadjustable interface controls to the lead-lag filter to effect an actualresponse of the position of the valve plug of the control valve.
 20. Thesystem of claim 14, wherein the lead-lag input filter is incommunication with a controller, said controller including programmingadapted to cause the lead-lag input filter to curtail movement of avalve stem of the control valve operatively coupled to the valve plug asthe valve plug approaches at least one of a valve seat and a travel stopof the control valve. 21-34. (canceled)
 35. A method for optimallytuning adjustment of a parameter of a control loop comprising: providinga lead-lag input filter in communication with an input of a controlloop; supplying an unamplified reference control signal to an input ofthe lead-lag input filter; providing at least one of a user interfaceand a controller in operable communication with the lead-lag inputfilter; and operating the user interface or controller to signal thelead-lag input filter to modify the reference control signal prior toapplication of the control signal to the input of the control loop. 36.The method of claim 35, and providing both the user interface and thecontroller in operable communication with the lead-lag input filter, andselecting among the user interface and the controller.
 37. The method ofclaim 36, wherein upon selecting the controller, at least partiallydisabling the user interface. 38-42. (canceled)